HVAC systems can be used to regulate the environment within an enclosure. Typically, an air blower is used to pull air from the enclosure into the HVAC system through ducts and push the air back into the enclosure through additional ducts after conditioning the air (e.g., heating, cooling or dehumidifying the air). Various types of HVAC systems, such as roof top units, may be used to provide conditioned air for enclosures. Additionally, different techniques may be employed for controlling the capacity of the HVAC systems. Two such techniques are variable air volume (VAV) and constant air volume (CAV).
In a VAV system, the temperature of the supply air for the enclosure is substantially constant and the air flow rate is varied to meet the thermal changes in the enclosure. A control function based on discharge air temperature may be used to control operation of a VAV HVAC system. As such, a sensor may be employed to determine the discharge air temperature of the HVAC system. Employing a discharge air temperature control allows an HVAC unit to typically deliver around 55° F. discharge supply air temperature.
In a CAV system, the air flow rate of the supply air is substantially constant and the temperature of the supply air is varied to meet thermal changes. In a CAV HVAC system, humidity may be more of a problem than with a VAV HVAC system. Thus, CAV HVAC systems may include a dehumidification function, such as employing reheat coils. Humiditrol® from Lennox Industries, Incorporated of Richardson, Tex., is an example of such a dehumidification function. Humiditrol® or other hot-gas reheat systems may reheat conditioned air to about 70° F. or more to provide discharged air that has a lower relative humidity, but not temperature, than the enclosure. This can be a result of the enclosure requiring more latent than sensible cooling.